Method for preparing a magnetic material by forging and magnetic material in powder form

ABSTRACT

The invention concerns a method for preparing a magnetic material by forging, characterised in that, in a first embodiment, it comprises the following steps; placing in a sheath an alloy based on at least one rare earth, at least one transition metal and at least one other element selected among boron and carbon; bringing the whole alloy to a temperature not less than 500° C.; forging the whole at a deformation speed of the material not less than 8 s −1 . After forging, it is possible to subject the resulting product to at least one annealing and hydridation then dehydridation, in another embodiment, it consists in starting with an alloy based on at least one rare earth and one transition metal and proceeding as in the first embodiment. After forging and, optionally, annealing, hydridation and dehydridation treatments, the resulting material is subjected to nitriding. The invention also concerns a magnetic material in power form, characterised in that has a coercivity not less than 9 kOe and retentivity not less than 9 kG.

The present invention relates to the preparation of a magnetic materialby forging and to a magnetic material in powder form.

Permanent magnets based on iron, boron and rare earths are well known.Their importance in the electrical or electronics industry is growing.

There are two main types of process for preparing these magnets. Thefirst makes use of powder metallurgy in order to prepare dense orsintered magnets.

Another process consists in melting an alloy and then quenching it on awheel, in annealing it and in hot pressing or encapsulating the powderthus obtained in a resin or a polymer. This process makes it possible toobtain bonded magnets. The powder and the magnet obtained byimplementing this process are usually isotropic. In order to obtain anisotropic powder or magnet, it is currently necessary to use expensiveprocesses which are inefficient or which give inadequate results.

There is therefore a need for a process for producing anisotropicproducts which is simpler to implement, is possibly more economical orhas an improved efficiency and which leads to products with satisfactoryor even improved properties.

The subject of the invention is the development of such a process.

To this end, the process of the invention for the preparation of amagnetic material is characterized in that it comprises the followingsteps:

an alloy based on at least one rare earth, on at least one transitionmetal and on at least one other element chosen from boron and carbon isplaced in a sheath;

the assembly is heated to a temperature of at least 500° C.;

the assembly is subjected to a forging operation with a strain rate ofthe material of at least 8 s⁻¹.

According to a second version, the process of the invention ischaracterized in that it comprises the following steps:

an alloy based on at least one rare earth and on at least one transitionmetal is placed in a sheath;

the assembly is heated to a temperature of at least 500° C.;

the assembly it subjected to a forging operation with a strain rate ofthe material of at least 8 s⁻¹;

the product after forging is subjected to a nitriding treatment.

The invention also relates to a magnetic material in powder form whichis characterized in that it has a coercivity of at least 9 kOe and aremanence of at least 9 kG.

Further characteristics, details and advantages of the invention willbecome even more apparent on reading the description which follows,together with the concrete but non-limiting examples intended toillustrate it.

The present invention applies, according to its first version, to thepreparation of magnetic materials based on at least one rare earth, onat least one transition metal and on at least one other element chosenfrom boron and carbon. The process of the invention therefore starts inthis case with alloys having the composition required for obtaining thedesired material. This composition may vary both in regard to the natureof its constituents and the respective proportions of them.

The invention involves alloys which comprise at least one rare earth andat least one transition metal and which also contain at least one otherelement chosen from boron and carbon. Such alloys are well known.

Throughout the description, the term “rare earth” should be understoodto mean one of the elements of the group formed by yttrium and theelements of the Periodic Table having an atomic number of between 57 and71 inclusive. The Periodic Table of the Elements to which reference ismade throughout the description is the one published in the Supplémentau Bulletin de la Société Chimique de France [Supplement to the Bulletinof the Chemical Society of France] No. 1 (January 1966).

The rare earth of the alloy may be neodymium or else praseodymium.Alloys based on several rare earths may be used. Mention may moreparticularly be made of alloys based on neodymium and praseodymium. Inthe case of an alloy of several rare earths, neodymium and/orpraseodymium may be the major component(s).

The term “transition elements” should be understood to mean the elementsof Columns IIIa to VIIa, VIII, Ib and IIb. In the present case, thesetransition elements may be more particularly those chosen from the groupcomprising iron, cobalt, copper, niobium, vanadium, molybdenum andnickel, it being possible for these elements to be taken alone or incombination. According to a preferred version, the transition element isiron or else iron in combination with at least one element of theaforementioned group, iron being the major component.

Apart from the aforementioned elements, the alloy may comprise additivessuch as gallium, aluminium, silicon, tin, bismuth, germanium, zirconiumor titanium, taken alone or in combination.

The respective proportions of rare earth, of transition metal and of theother aforementioned element may vary widely. Thus, the rare-earthcontent may be at least 1% (the percentages given here are atomicpercentages) and it may vary between approximately 1% and 30%, moreparticularly between approximately 1% and 20%. The content of the thirdelement, especially boron, may be at least 0.5% and it may vary betweenapproximately 0.5 and 30%, more particularly between approximately 2 and10%. In the case of the additives, their content may be at least 0.05%and it may vary from approximately 0.05 to 5%.

By way of examples of alloys, mention may most particularly be made ofneodymium/iron/boron alloys, especially those which also comprisecopper. Mention may also be made, as alloys which can be used moreparticularly in the context of the present invention, of those whichhave a phase satisfying the formula RE₂Fe₁₄B, RE denoting at least onerare earth, most particularly neodymium.

The invention also applies, according to its second version, to thepreparation of magnetic materials based on at least one rare earth, onat least one transition metal and on nitrogen. The process used in thiscase starts with alloys having the composition, in terms of rare earthand of transition metal, required to obtain the desired material.Everything that was stated above in regard to the rare earth, thetransition element and the optional additives also applies in this ease.However, mention may more particularly be made of alloys based onsamarium and iron, from which alloys magnetic materials based onsamarium, iron and nitrogen will be obtained.

It will be noted that the alloys used as starting products do not havethe properties of magnets, or do so very slightly. In particular, theyhave a very small or zero coercivity and exhibit very little or noanisotropy. The alloys that are used generally consist of mostly large,single-crystal grains with a size of at least approximately 10 μm. Here,and for the entire description, sizes are measured by SEM. The alloysmay be in bulk form or in powder form. The alloys are generallyheterogenous with regard to the grain size, to the nature of the phasesand, in the case of a powder, to the particle size.

Prior to the treatment of the invention, the alloy may be annealed at atemperature of at least 500° C. in an inert atmosphere.

The alloy as described above is placed in a sheath. Advantageously, acylindrical sheath is used. The height of this sheath is preferably atleast equal to the height of the alloy to be treated. Its wall thicknessis chosen in such a way that it does not burst during forging, but thisthickness must remain relatively small. The material of which the sheathis composed must be as plastic as possible at the temperature at whichforging takes place. Generally, a metal sheath is used. Preferably, thesheath is made of steel.

The alloy may be introduced into the sheath by the molten alloy beingcast into it, or by any mechanical means starting with an ingot or withpowder.

The alloy/sheath assembly is then heated to a temperature of at least500° C. The maximum temperature not to be exceeded is that above whichthere is a risk of significant melting of the grains or particles of thealloy occurring. This temperature is more specifically between 600° C.and 1100° C. more particularly between 800° C. and 1000° C. The alloy isheated to the indicated temperature in an inert atmosphere, for examplein argon.

However, it is possible to carry out the process in a sealed casing.This means that, once the alloy has been placed in the sheath, the topand bottom of the assembly formed by the sheath and the alloy are sealedby means of a cover made of a material which may be of the same natureas that of the sheath, the cover being welded to the sheath. The alloyis thus isolated from the outside and it may be heated to the requiredtemperature without it being necessary to work in an inert atmosphere.

The next step of the process of the invention consists in subjecting thealloy in the sheath to a forging operation. The forging consists of apercussion: the alloy/sheath assembly is given one or more blows by aforge hammer. Forging takes place on the alloy/sheath assembly at thetemperature indicated above. When the sheath is not sealed, thealloy/sheath assembly is placed in a sealed chamber surrounding theanvil of the forge. This chamber is connected to a source of inert gasand comprises an opening through which the forge hammer can pass via aseal.

Generally, the number of hammer blows is between 1 and 10.

The mechanical power of the hammer blow must be such that theconstituent grains of the alloy break. It must also be such that some ofthis power serves to heat the material, allowing several successiveforging treatments, without heating up the alloy from the outside. Thus,this power may, for example, be at least about 1 kilowatt per gramme ofmaterial (kW/g), more particularly at least 5 kW/g. Such a powercorresponds to a strain rate of the material of at least 8 s⁻¹,especially at least 10 s⁻¹ more particularly at least 50 s⁻¹ and evenmore particularly at least 100 s⁻¹. The strain rate of the material isdefined by the expression (dh/h)/dt, dh/h denoting the (initialheight−final height)/initial height ratio, the height being that of thealloy/sheath assembly, dt denoting the compression time, which is equalto dh/(v/2), v being the speed of the hammer at the moment of impact andv/2 being regarded, to a first approximation, as the average speedduring compression, which average speed may in fact be defined as the(initial speed−final speed)/2 ratio, i.e. (v-0)/2.

Such a power corresponds to devices in which the hammer speed is atleast 0.3 m/s, especially 0.5 m/s, more particularly at least 1 m/s andeven more particularly at least 4 m/s.

Forging may be carried out with a reduction ratio of at least 2. Thereduction ratio is defined by the initial height (before forging)/finalheight (after forging) ratio of the alloy/sheath assembly. This ratiomay be more particularly at least 5.

According to a preferred embodiment of the invention, forging is carriedout in a direction perpendicular to an easy growth axis of thecrystallites of the alloy. In the case of the Nd₂Fe₁₄B phase, this easygrowth axis is the a or b axis of the tetragonal unit cell. In thiscase, forging allows the c axes to move from an equatorial distributionto an approximately unidirectional distribution.

After forging, the product obtained is in the form of a flat cylinder,or possibly in the form of a capsule when a sealed casing has been used,as described above, the internal part of which contains the startingmetal alloy and the peripheral or external part of which comprises thestarting sheath. The alloy now consists of single-crystal grains, theaverage size of which is at most 30 μm, more particularly at most 10 μm.The alloy has a coercivity and is anisotropic. The magnetization axesare aligned parallel to the forging direction.

According to the second version of the invention, and for the purpose ofobtaining a magnetic material based on at least one rare earth, on atleast one transition metal and on nitrogen, the product after forging issubjected to a nitriding treatment. The nitriding treatment is carriedout in a known manner. The nitrogen content of the material obtained maybe of the same order as that given above in the case of boron, moreparticularly it may be between 2 and 15%.

The process of the invention may furthermore comprise, after the forgingstep, other, complementary steps involving treatments which will bedescribed below. In the case of the preparation of a magnetic materialbased on at least one rare earth, on at least one transition metal andon nitrogen, the preparation including a nitriding step, thecomplementary treatments are preferably carried out before thisnitriding step.

The various complementary treatments which will now be described may becarried out in any order.

By way of complementary treatment, it is thus possible for the productafter forging to be subjected to at least one annealing treatment inorder to improve its magnetic properties.

Various types of annealing treatment may be envisaged. A first type iscarried out at a temperature which may be between 700° C. and 1100° C.The treatment is preferably carried out in an inert atmosphere, forexample in argon. The duration of the treatment may be between a fewminutes and a few hours.

Another type of annealing treatment may be carried out at a temperatureof between 400° C. and 700° C., also preferably in an inert atmosphereof the argon type. The duration of the treatment may be between a fewminutes and a few hours.

Of course, it is quite possible to carry out one or more annealingtreatments of the same type or of a different type. For example, atreatment of the first type mentioned above may be carried out followedby a second treatment of the second type.

As another complementary treatment, it is also possible to provide ahydrogen cracking process for the purpose of obtaining a powder havingmagnetic properties similar to those of the bulk product. Thus, thematerial obtained after forging, and optionally after at least oneannealing treatment, may be subjected to a hydriding treatment, so as toobtain a hydride of the alloy, and then a dehydriding treatment.

Hydriding and dehydriding treatments are known. The material may bedehydrided in a hydrogen atmosphere (for example, at at least 0.1 MPa)at room temperature or else by thermally activating the material in anatmosphere containing hydrogen. For example, the material may bethermally activated up to a temperature of lens than 500° C., preferablyless than 300° C. The hydrided material may be dehydrided by beingheated at a temperature of at least 500° C. in vacuo. The temperatureand the heating time are chosen so that the material is completelydehydrided. Optionally, the dehydriding treatment may be followed by anannealing treatment of the first and/or second type mentioned above.

After this treatment, a material in powder form having useful magneticproperties is obtained. Thus, this material has a coercivity of at least9 kOe, more particularly at least 9.5 kOe and even more particularly atleast 10 kOe, in combination with a remanence of at least 9 kG, moreparticularly of at least 9.5 kG and even more particularly of at least10 kG. The material may have each of the coercivity values given abovein combination with each of the remanence values, also given above, forexample a coercivity of 9 kOe in combination with a remanence of 9.5 kG.The material has a crystalline texture, making it magneticallyanisotropic. The constituent particles of the powder themselves consistnot of just one single-crystal grain but of several single-crystalgrains having an average size of at least 0.1 μm. Thus, for example, theparticles may have a size of a few tens of microns, especially betweenapproximately 10 μm and approximately 200 μm, more particularly betweenapproximately 10 μm and approximately 100 μm, and may consist of aboutten grains each being a few microns in size.

With regard to its composition, the material consists of the constituentelements which were given above for the alloy and that which wasdescribed in that case also applies in this case, the material beingbased, in particular, on at least one rare earth, on at least onetransition metal and on at least one other element chosen from boron,carbon and nitrogen.

Examples will now be given.

The alloy used satisfies the formulaNd_(15.3)Fe_(76.8)B_(4.9)Cu_(1.5)Al_(1.5) in the case of Examples 1 and2, the formula Nd_(15.5)Fe₇₈B₅Cu_(1.5) in the case of Example 3 and theformula Nd_(15.3)Fe_(76.9)B_(4.9)Cu_(1.5)Nb_(0.5)Al_(0.9) in the case ofExample 4.

Tests are carried out on a cylindrical steel sheath. In certain cases,the alloy is subjected to two hammer blows (first forging and secondforging).

Table 1 gives the characteristics of the starting material, Tables 2 and3 give the forging conditions and Table 4 gives the magnetic propertiesof the bulk materials obtained.

TABLE 1 Mass of the Thickness of sheath and alloy Diameter Height thesheath Example (g) (mm) (mm) (mm) 1 20.18 12 25 2 2 15.76 12 20 1 320.31 12 25 1 4 19.98 12 24.5 1

TABLE 2 T₁ T₂ E Example (° C) (° C) (s⁻¹) Tr₁ Tr₂ 1 980 890 95.0 4.396.25 2 1060 — 112.5 5.90 — 3 995 — 95.6 6.00 — 4 1000 — 92 6 — T₁:temperature during the first forging T₂: temperature during the secondforging E: strain rate during the first forging Tr₁: reduction ratioafter the first forging Tr₂: total reduction ratio after the secondforging

TABLE 3 V₁ V₂ P₁ P₂ Example (m/s) (m/s) (kW/g) (kW/g) 1 4.75 4 10.3 70 24.54 — 13.9 — 3 4.78 — 9.8 — 4 4.48 — 8.3 — V₂: hammer speed during thefirst forging V₂: hammer speed during the second forging P₁: mechanicalpower of the first hammer blow P₂: mechanical power of the second hammerblow

TABLE 4 Coercivity Remanence Energy Hc Br product Example kOe kA/m kG TMGOe kJ/m³ 1 9.5 756 10 1 17.5 139 2 10.0 796 10 1 16 127 3 9.5 756 10 117.5 139 4 10.1 804 9.9 0.99 21 167

The remanence values given in Table 4 show that the products areanisotropic.

What is claimed is:
 1. Process for the preparation of a magneticmaterial, comprising the following steps: placing an alloy based on atleast one rare earth, at least one transition metal and at least oneother element selected from the group consisting of boron and carbon ina sheath to form an assembly; heating the assembly to a temperature ofat least 500° C.; and subjecting the assembly to a forging operationwith a strain rate of the material of at least 8 s⁻¹, wherein theforging is carried out in a direction perpendicular to an easy growthaxis of the crystallites of the alloy.
 2. Process for the preparation ofa magnetic material based on at least one rare earth, at least onetransition metal, and nitrogen, comprising the following steps: placingan alloy based on at least one rare earth and at least one transitionmetal in a steel sheath to form an assembly; heating the assembly to atemperature of at least 500° C.; subjecting the assembly to a forgingoperation with a strain rate of the material of at least 8 s⁻¹; andsubjecting the alloy after forging to a nitriding treatment.
 3. Processaccording to claim 1, wherein the forging is carried out with a strainrate of the material of at least 10 s⁻¹.
 4. Process according to claim1, wherein the forging is carried out with a reduction ratio of at least2.
 5. Process according to claim 1, wherein the rare earth comprisesneodymium.
 6. Process according to claim 1, wherein the alloy is basedon iron.
 7. Process according to claim 1, wherein the at least one otherelement is boron.
 8. Process for the preparation of a magnetic material,comprising the following steps: placing an alloy based on at least onerare earth, at least one transition metal and at least one other elementselected from the group consisting of boron and carbon in a sheath toform an assembly; heating the assembly to a temperature of at least 500°C.; and subjecting the assembly to a forging operation with a strainrate of the material of at least 8 s⁻¹, wherein the alloy also comprisescopper.
 9. Process according to claim 1, wherein the sheath is made ofsteel.
 10. Process according to claim 8, wherein the sheath is made ofsteel.
 11. Process for the preparation of a magnetic material,comprising the following steps: placing an alloy based on at least onerare earth, at least one transition metal and at least one other elementselected from the group consisting of boron and carbon in a sheath toform an assembly; heating the assembly to a temperature of at least 500°C.; and subjecting the assembly to a forging operation with a strainrate of the material of at least 8 s⁻¹, wherein the material obtainedafter forging is subjected to at least one annealing treatment. 12.Process for the preparation of a magnetic material, comprising thefollowing steps: placing an alloy based on at least one rare earth, atleast one transition metal and at least one other element selected fromthe group consisting of boron and carbon in a sheath to form anassembly; heating the assembly to a temperature of at least 500° C.; andsubjecting the assembly to a forging operation with a strain rate of thematerial of at least 8 s⁻¹, wherein the material obtained after forging,and, optionally, after at least one annealing treatment, is subjected toa hydriding treatment and then to a dehydriding treatment, so as tochange the material into powder form.
 13. Magnetic material in powderform, based on at least one rare earth, at least one transition metaland at least one other element selected from the group consisting ofboron and carbon obtained by the process according to claim 12, having acoercivity of at least 9 kOe and a remanence of at least 9 kG. 14.Material according to claim 13, in the form of a powder consisting of 10to 200 μm particles.
 15. Material according to claim 13, in the form ofa powder, the particles of which comprise single-grain crystals havingan average size of at least 0.1 μm.
 16. Material according to claim 13,which is magnetically anisotropic.
 17. Process according to claim 1,herein the forging is carried out with a strain rate of the material ofat least 50 s⁻¹.
 18. Process according to claim 1, wherein the forgingis carried out with a strain rate of the material of at least 100 s⁻¹.19. Process according to claim 2, wherein the forging is carried outwith a strain rate of the material of at least 10 s⁻¹.
 20. Processaccording to claim 2, wherein the forging is carried out with a strainrate of the material of at least 50 s⁻¹.
 21. Process according to claim2, wherein the forging is carried out with a strain rate of the materialof at least 100 s⁻¹.
 22. Process according to claim 2, wherein theforging is carried out with a reduction ratio of at least
 2. 23. Processaccording to claim 2, wherein the forging is carried out in a directionperpendicular to an easy growth axis of the crystallites of the alloy.24. Process according to claim 2, wherein the rare earth comprisessamarium.
 25. Process according to claim 2, wherein the alloy is basedon iron.
 26. Process according to claim 2, wherein the material obtainedafter forging, and before the nitriding treatment, is subjected to atleast one annealing treatment.
 27. Process according to claim 2, whereinthe material obtained after forging, and, optionally, after at least oneannealing treatment, is subjected to a hydriding treatment and then to adehydriding treatment, so as to change the material into powder form,and followed by a nitriding treatment.
 28. Process for the preparationof a magnetic material, comprising the following steps: placing an alloybased on at least one rare earth, at least one transition metal and atleast one other element selected from the group consisting of boron andcarbon in a sheath to form an assembly; heating the assembly to atemperature of at least 500° C.; and subjecting the assembly to aforging operation with a strain rate of the material of at least 8 s⁻¹,wherein the alloy prior to said forging comprises mostly large, singlecrystal grains with a size of at least approximately 10 μm.
 29. Materialaccording to claim 13, in the form of a powder, the particles of whichcomprise single-grain crystals having an average size of at least 0.1 μmand the particles comprise particles having a size between 10 and 200μm.